There are no apparatuses that are adapted to estimate the remainder of the lifetime of a xenon lamp used in a light-fastness tester as a light source, and then displaying the estimated remainder i.e. the time during which the xenon lamp can still be used, in terms of hours. The remainder of the lifetime of a xenon lamp has generally been estimated on the basis of empirical facts only. For example, the remainder of the lifetime of a xenon lamp which has a maximum usable life of 2000 hours, and which has already been used for 500 hours, is estimated to have a remaining lifetime of 1500 hours by subtracting 500 hours from 2000 hours.
In such a case, a cumulative time measuring instrument is used for conveniently memorizing the time which the xenon lamp in question has been used. When the xenon lamp in question starts being used, the pointer of the cumulative time measuring instrument is set to zero so that the time of use of the xenon lamp is cumulatively displayed thereon.
It is essential for a light-fastness tester to apply a constant irradiance of the light emitted from a light source at all times on the surface of a sample to be tested. Since the irradiance of the light applied from a xenon lamp to a sample decreases as the time of use thereof increases, it is necessary that the level of discharge power of the xenon lamp be varied so as to maintain a predetermined irradiance.
An automatic xenon lamp energy regulator provided with a light-receiving sensor in a sample position and adapted to automatically control the discharge power of a xenon lamp for the purpose of maintaining the irradiance of the light emitted therefrom at a constant level on the surface of a sample has heretofore been used.
Since the xenon lamps are used in combination with optical glass filters, the lifetimes of the lamps differ with the condition of deterioration of the optical glass filters and the variation in the performance of the lamps, so that it is difficult to estimate accurately the remainder of the lifetime of each lamp.
A test carried out by a light-fastness tester may be conducted for as long as 2000 hours or more in some cases where the quality of the object being tested is high. When a xenon lamp fails at a midnight or on a holiday during the operation of the light-fastness tester, the sample being subjected to a long test time is wasted in many cases and causes a great loss. Therefore, it is important to be able to estimate accurately the remainder of the lifetime of the xenon lamp to avoid interruptions in or permature ending of light-fastness tests.